Noteworthy article on AppleInsider.com yesterday (AppleInsider | iOS 'stickiness' grows as average Apple user has $100 in content per device): the average iOS device (iPhone + iPad + iPod) has roughly $100 of paid content on it. That number represents all content downloadable through iTunes: applications, music, movies and books. More impressively, the average has risen to ~$100 from ~$80 in the last year - and that is as 75m new iOS devices came onto the network.
Not only is the average high and ramping significantly - it suggests a very high switching cost. To leave Apple, you are losing $100 of paid content and - in many cases - are unable to even replace that content on other networks / OS's because of limited inventory.
This is strengthened by Apple's new iCloud - which allows users to easily transfer content (apps, music, videos, etc) from device to device. This previously was a very painful activity... and, at times, it actually deterred me from buying some content. That is solved with iOS5 and I find myself buying more content because it is instantly accessible on all of my devices.